Wireless communication channels suffer from fading, or loss of signal, due to changes in the propagation environment of the wireless signal. Some types of fading, such as Rayleigh fading, can be highly localized in nature. Furthermore, wireless communication systems are often limited in the amount bandwidth that can be used, due to practical restrictions on the electronics that are used, or due to licensing and regulatory restrictions.
Multiple-input and multiple-output, or MIMO, refers to the use of multiple antennas at the transmitter and the receiver end of a wireless link. MIMO technology may offer significant increases in data throughput and/or transmission range without the need for additional bandwidth or transmit power. It can achieve this due to the ability of MIMO to obtain higher spectral efficiency (more bits per second per hertz of bandwidth) and/or reduced fading.
MIMO based systems allow the use of a variety of coding techniques that take advantage of the presence of multiple transmit and receive antennas. For example, wireless communications performed over a MIMO channel can use beamforming, spatial multiplexing and/or diversity coding techniques.
Beamforming involves transmitting the same signal on each of the transmit antennas with appropriate complex (i.e., gain and phase) weighting such that the signal power is increased at the receiver input. The benefits of beamforming are to increase the signal gain from constructive interference and to reduce the multipath fading effect.
In spatial multiplexing, a high data rate signal is split into multiple lower data rate streams, and each stream is transmitted from a different transmit antenna in the same frequency channel. The receiver separates the received streams and combines the received data streams into a single receive stream, thereby increasing channel capacity.
In diversity coding methods, a single stream is transmitted, but the signal is coded using space-time coding techniques so that the signal emitted from each of the transmit antennas is substantially orthogonal. Diversity coding exploits the independent fading in the multiple antenna links to enhance signal diversity.
To increase performance of a MIMO system, it is desirable for fading on the wireless links between the transmit and receive antennas to be uncorrelated. That is, it is desirable for there to be a low statistical correlation between fading experienced at one antenna and fading experienced at another antenna.
Correlation between antennas can be reduced by causing the antennas to have different polarizations, i.e. sending and receiving signals with orthogonal polarizations. Furthermore, antennas for MIMO systems may utilize spatial separation, or physical separation, to reduce correlation between antennas. Either of these approaches can be unsatisfactory for handheld mobile devices, however, as it is generally desirable for the handheld devices to have compact antennas.